The Stirling cycle engine is a reciprocating heat engine which operates by transferring heat from an external source into the cylinder through a solid wall, rather than by exploding a fuel-air mixture within the cylinder. This is known as an external combustion engine (although the heat may come from sources other than burning fuel such as solar or nuclear), as opposed to the familiar internal combustion engine. The heat is transferred to an internal gaseous working fluid which is sealed within the cylinder and undergoes closed cycle heating, expansion, cooling, and compression, alternately flowing back and forth through thermal storage device known as a regenerator.
As more fully described in earlier Moscrip U.S. Pat. Nos. 4,413,473; 4,413,474; 4,413,475; and 4,429,732; heat is supplied continuously in one part of the machine, called the heater, and is removed continuously in another part of the machine called the cooler. The regenerator picks up heat when the gas goes from the hot side to the cold side and gives up heat to the gas when it is moving in the opposite direction. The simplest Stirling engine configuration, known as the Alpha configuration, is one in which there are one or more pairs of sealed pistons, one, a compression piston and the other, an expansion piston. The motion of one piston leads the motion of the other piston by a mechanical phase angle of approximately ninety degrees. The phase angle is often prescribed by the design of a crankshaft, swashplate, or other mechanical element.
The continuous burning of fuel attainable in engines employing external as opposed to internal combustion permits the achievement of high temperatures and other conditions which result in complete combustion. This in turn leads to exceptionally low levels of undesirable components in the exhaust emissions. Because the pistons reciprocate with smooth harmonic motion, and because there are no abrupt periodic detonations inside the cylinder, the operation of Stirling engines can be unusually quiet. Because the heat required can be obtained from virtually any source, Stirling engines can be designed to run on a large variety of fuels or multiple fuels. And because of their inherently high efficiency and low exhaust emissions, economical operation is possible.
Surprisingly, the first Stirling engine was patented in 1816 by a Scottish clergyman, Robert Stirling. These machines were known as hot air engines throughout the 19th century, during which they were improved by a number of famous engineers of the period, among them John Ericsson, who built the Monitor during the Civil War. By the 1920's the internal combustion engine, as well as the steam engine and the electric motor, had all but eliminated the Stirling engine from the marketplace and consigned it to the world's history and technology museums.
The hot air engine might have been forever relegated to the museum if the Dutch firm, N.V. Philips, had not taken an interest in such machines in 1937. A manufacturer of portable radio equipment, N.V. Philips was interested in the engine for its potential application as a compact and quiet power source which, because of its spark-free operation, would create no radio interference. These efforts led to the development of contemporary Stirling cycle engines which utilize either hydrogen or helium as the working fluid and which incorporate modern developments in materials technology.
Today's Stirling engines exhibit excellent thermal efficiency, multiple fuel capability, quiet operation, and favorable torque characteristics. Modern designs are the direct result of increased operating temperatures and heat transfer rates, decreased complexity of mechanical arrangements, and the availability of superior materials. But they remain predominately confined to the laboratory, with the exception of a few specialized and high-dollar market applications such as space power and cryogenic coolers, because existing machines are invariably too complex, costly, and unreliable to compete with the available alternatives in more commonplace fields.
Conventional mechanical arrangements tend to borrow heavily from traditional internal combustion engine designs, which are incompatible with optimum Stirling engine design. This is because the Stirling machine needs different hardware to introduce, to control, and to eliminate heat. The conventional use of hydrogen and helium as gaseous working fluids imposes the additional expense of exotic materials and coatings, because these gases are difficult to contain and because they make ordinary engine materials brittle. In addition, their use necessitates the incorporation of intricate seals in the design, increasing friction, decreasing reliability, and adding to the overall production cost.
The mechanical arrangements of Stirling engines are generally divided into three groups known as the Alpha, Beta, and Gamma arrangements, after D.W. Kirkley (Kirkley, D.W., 1962; "Determination of the Optimum Configuration for a Stirling Engine", J. Mech. Eng. Sci., 4:204-12.). Alpha engines have pairs of sealed pistons in separate cylinders which are connected in series by a heater, a regenerator, and a cooler. Both Beta and Gamma engines are defined by the use of a classic piston-displacer arrangement, the Beta engine having both the displacer and the piston in the same cylinder, while the Gamma engine uses separate cylinders (West, C.D., 1986; "Principles and Applications of Stirling Engines" New York: Van Nostrand Reinhold Company.).
The principal distinction between a piston and a displacer is that pistons are, and displacers are not, provided with a nominally gas-tight fluid seal to prevent the passage of gas from one side to the other during normal operation. Thus there is usually a more substantial pressure gradient created by the operation of a piston than by that of a displacer. A displacer does no work on the gas in general, but merely displaces it from one place to another whereas work is done on the gas by a piston, or on a piston by the gas, as the piston moves within the cylinder.
The mechanical arrangements of Stirling engines are also often broadly divided into two primary groups, namely kinematic and free piston engines (West, C.D., 1986; "Principles and Applications of Stirling Engines" New York: Van Nostrand Reinhold Company Urieli, I., and Berchowitz, D.M., 1984; "Stirling Cycle Engine Analysis" Bristol: Adam Hilger Ltd ). The term kinematic drive is commonly used to describe any arrangement of cranks, connecting rods, swash plates, cams, and other mechanical dynamic components which serve to constrain the motion of either pistons or displacers within a prescribed phase relationship, producing useful output power by conventional mechanical means such as a rotating shaft. The term free piston drive commonly describes Stirling engines wherein the inherent working fluid pressure variations and other thermodynamic and gasdynamic forces are employed by a given design to achieve the appropriate phase angle, work being removed by a device such as a linear alternator or a hydraulic pump.
The invention of the basic free piston Stirling engine in the early 1960s is generally attributed to William T. Beale (Beale, W.T., 1969; "Free-piston Stirling Engines--Some Model Tests and Simulations", SAE Paper No. 690230 Beale, W T , 1971; "Stirling Cycle Type Thermal Device", U.S. Pat. No. 3,552,120. Beale, W.T. and Scheck, C.G., 1986; "Electromechanical Transducer Particularly Suitable for a Linear Alternator Driven by a Free-Piston Stirling Engine", U.S. Pat. No. 4,623,808). The independent discovery of similar engines is attributed to E.H. Cooke-Yarborough and C.D. West of the Atomic Energy Research Establishment, Harwell, England (Cooke-Yarborough, E.H., 1967; "A Proposal for a Heat-Powered Nonrotating Electrical Alternator", Harwell Memorandum AERE-M881 UK AERE. Cooke-Yarborough, E H., 1970; "Heat Engines", U.S. Pat. No. 3,548,589. Cooke-Yarborough, E.H.; Franklin, E.; Geisow, J.; Howlett, R.; and West, C.D.; 1974; "Harwell Thermo-Mechanical Generator", Proc 9th IECEC, Paper No. 749156.). G.M. Benson also made important contributions to this segment of the prior art and patented many novel free piston engines. Others have since been working on various modifications of and improvements to the original free piston design concepts (Benson, G M., 1977; "Thermal Oscillators", Proc. 12th IECEC, Paper No. 779247. Walker, G. and Senft, J R., 1985; "Free Piston Stirling Engines", New York, Heidelberg, Berlin: Springer-Verlag. Walker, G., 1980; "Stirling Engines", Oxford: Clarendon Press Vincent, R.J.; Rifkin, D.W.; and Benson, G.M.; 1980; "Analysis and Design of Free-Piston Stirling Engines--Dynamics and Thermodynamics", Proc 15th IECEC, Paper No. 809334.). Free piston engines are undergoing intensive investigation by NASA for space power applications because of their potential for long life, high reliability and efficiency, low vibration, and relatively low noise (Slaby, J.G., 1985; "Overview of Free-Piston Stirling Technology at the NASA Lewis Research Center", NASA TM-87156.).
Virtually all of the existing free piston engine designs currently being developed incorporate piston and displacer arrangements, i.e., they are either Beta or Gamma type machines. Free piston engines of the Alpha type are unknown, although they might be expected to embody less complicated designs and to have therefore a lower production cost. However, the present invention is believed to be essentially different from and superior to either kinematic engines or free piston engines by virtue of the fact that it relies upon a unique combination of mechanical dynamic, thermodynamic and gasdynamic, and electrodynamic and magnetodynamic forces to maintain the prescribed phase relationship among the pistons of an Alpha type Stirling cycle machine. While it lacks the complex and costly mechanical components of kinematic engines, the present invention's pistons do not execute free piston motion but are strongly constrained by electrodynamic and magnetodynamic forces in a quasi free piston manner.